In recent years large metal structures, in particular steel structures, have been built with such large dimensions and have been used under so severe circumstances that the need for detection and localization of defects therein, e.g. cracks or fractures, has become urgent in many applications. Examples in this respect are large vessels and offshore petroleum production structures which occasionally have met with accidents that have increased the need for more accurate monitoring systems.
Various measuring principles of such monitoring systems have been suggested, i.e. methods based on vibration analysis, acoustic emission, ultrasonic systems, registration of magnetic fields as well as visual inspection. So far these methods have not been adequate in this connection, mainly because of the great strains to which the monitoring equipment can be exposed to, in particular under water. In addition to being resistant to such strains, it is also important that the monitoring systems do not entail prohibitive installation and maintenance costs, since the components or areas which are to be monitored often have very large dimensions. Systems that, for example, are based on the use of a number of transducers, e.g. for detection of vibration, acoustic emission or ultrasonic signals, may involve the mounting of a large number of transducers, each one representing a comparatively high expense and a risk of failure.
Another previously disclosed method for examination of cracks in structural parts or components is based on the measurement of the electric field that is produced in the structure. This so-called potential drop method is used for detailed examination of a crack that has been localized beforehand. The size or depth of the crack is determined by means of contact points on each side of the crack, between which the voltage is measured. The supplied current is either DC or AC with a low frequency. Different versions of the potential drop method have been published in German Patent Application No. 25 57 658 and in United Kingdom Patent Specification No. 804.323. In the former publication, a high frequency current supply is used, and the examination comprises measurements of the drop of potential as a function of the frequency. In the latter case, which in particular has been reported to concern surface cracks, a radio frequency potential is measured, which potential occurs between two separate electrodes that are moved on the surface of the structure while an oscillating electric current is supplied thereto from a source of radio frequency. Such an arrangement with movable electrodes which are to be guided all over the monitoring area cannot, however, be used for the purpose mentioned above.